Phosphor blend for an LED light source and LED light source incorporating same

ABSTRACT

A phosphor blend for an LED light source is provided wherein the phosphor blend comprises from about 7 to about 12 weight percent of a cerium-activated yttrium aluminum garnet phosphor, from about 3 to about 6 weight percent of a europium-activated strontium calcium silicon nitride phosphor, from about 15 to about 20 weight percent of a europium-activated calcium silicon nitride phosphor, and from about 55 to about 80 weight percent of a europium-activated calcium magnesium chlorosilicate phosphor. An LED light source in accordance with this invention has a B:G:R ratio for a 3200K tungsten balanced color film of X:Y:Z when directly exposed through a nominal photographic lens, wherein X, Y and Z each have a value from 0.90 to 1.10.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/234,518, filed Aug. 17, 2009. This application is related toco-pending application Ser. No. 13/388,953, filed Feb. 3, 2012.

TECHNICAL FIELD

This invention relates to lighting for photography and cinematography,especially to the use of light emitting diodes (LEDs) for theseapplications.

BACKGROUND OF THE INVENTION

Lighting for the film industry started with sunlight about the beginningof the last century and thereafter evolved through the carbon arc to amix of incandescent lamps, medium arc high intensity discharge lamps(e.g., HMI), and more recently, fluorescent lighting. To a greater orlesser extent, these light sources all required large amounts ofelectric power and generate large amounts of heat that is undesirableduring film production. High power consumption is an especiallydifficult problem for location production where a supply of electricpower may be limited unless portable electric generating plants are madeavailable which can increase the cost of production.

In addition, the films that have been developed over the years have beenstandardized to a degree to have responses based on the spectralcharacteristics of specific light sources. In particular, two widelyused films have based their responses on a incandescent 3200K tungstensource and a standard 5500 K daylight source as exemplified by KODAKColor Negative Film 5219 and KODAK Color Negative Film 5205,respectively. It is important therefore that any new light sources whichmight be developed for photography or cinematography have appropriatespectral power distributions for use with these films.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is a another object of the invention to provide a light source basedon LEDs with the major objectives of reducing heat in the vicinity ofthe light sources and, more importantly, reducing the power required tooperate such lighting.

It is a further object of the invention to provide a light source havinga spectral power distribution appropriate for a 3200K tungsten film.

In accordance with an aspect of the invention, there is provided aphosphor blend for an LED light source comprising from about 7 to about12 weight percent of a cerium-activated yttrium aluminum garnetphosphor, from about 3 to about 6 weight percent of a europium-activatedstrontium calcium silicon nitride phosphor, from about 15 to about 20weight percent of a europium-activated calcium silicon nitride phosphor,and from about 55 to about 80 weight percent of a europium-activatedcalcium magnesium chlorosilicate phosphor.

Preferably, the phosphor blend comprises from about 8 to about 10.5weight percent of the cerium-activated yttrium aluminum garnet phosphor,from about 4 to about 5 weight percent of the europium-activatedstrontium calcium silicon nitride phosphor, from about 16.5 to about18.5 weight percent of the europium-activated calcium silicon nitridephosphor, and from about 65 to about 75 weight percent of theeuropium-activated calcium magnesium chlorosilicate phosphor.

More preferably. the phosphor blend of comprises about 10 weight percentof the cerium-activated yttrium aluminum garnet phosphor, about 4.5weight percent of the europium-activated strontium calcium siliconnitride phosphor, about 17.5 weight percent of the europium-activatedcalcium silicon nitride phosphor, and about 68 weight percent of theeuropium-activated calcium magnesium chlorosilicate phosphor.

In accordance with another aspect of the invention, there is provided anLED light source having a B:G:R ratio for a 3200K tungsten balancedcolor film of X:Y:Z when directly exposed through a nominal photographiclens, wherein X, Y and Z each have a value from 0.90 to 1.10.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the three film response sensitivities for 3200 Ktungsten film.

FIG. 2 shows the relative spectral power distribution of a 3200 Kincandescent light source.

FIG. 3 shows the average lens spectral transmittance used in thestandard ANSI PH2.3-1972.

FIG. 4 shows a spectrum of an embodiment of an LED light sourceaccording to this invention for use with 3200 K tungsten balanced colorfilm.

FIG. 5 shows the chromaticity point of the LED light source on the 1931CIE Chromaticity Diagram.

FIG. 6 shows the individual emission curves for the four phosphorscomprising the phosphor blend according to this invention.

FIG. 7 is a top view of an array of LED light sources according to thisinvention.

FIG. 8 is a cross-sectional illustration of one of the LED light sourcesshown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment, the LED light source comprises eitherdiscrete LED elements or arrays of nominally identical elements that canbe arranged in various ways to form a complete light source. The colorquality as described by the spectral power distribution is balanced tosatisfy the exposure requirements of 3200 K tungsten film as exemplifiedby KODAK Color Negative Film 5219 although the LED source generally willfunction satisfactorily with other films nominally balanced for 3200 Ktungsten. The sine qua non is that non-selectively reflecting surfaceswill be reproduced achromatically by the developed film. It will benoted that trichromatic films of this type cannot ever reproduce allpossible colored reflective surfaces identically to their direct visualappearances irrespective of the light source spectral power distributionthat is used. This well-known limitation is described in KodakPublication No. E-73, Why a Color May Not Reproduce Correctly(312-10-78-DX, 1978).

3200 K tungsten film is nominally balanced to give proper colorreproduction with an incandescent lamp radiating at a color temperature(CIE 15:2004) of 3200 K. However, certain spectral power distributionsthat differ from this one can be made acceptable. Note that anacceptable alternative does not necessarily have to have a correlatedcolor temperature (CIE 15:2004) of 3200 K. Regardless of the actualcorrelated color temperature of the LED source, it is identified as a3200 K photographic light indicating that it is to be used with 3200 Ktungsten balanced film. Further, it is well-known that handheldphotographic color temperature meters may not indicate either 3200 K orthe actual correlated color temperature of the LED source. The lightdoes not have to have any particular visual appearance in terms of colorquality, and it does not have to meet any particular color qualitymetric for visual purposes. Parenthetically, we will note that theGeneral Color Rendering Index (CIE 13.3-1995) of an acceptable light forthis application commonly will fall in an acceptable range of values forcommercial lighting lamps, i.e., roughly 70 or higher. The basicrequirement is that the photographic system reproduces color in a properand acceptable manner.

The method for designing the light source is to control three metricvalues proportional to the three components of the film response whenthe film is directly exposed to the light source spectrum. The threecomponents are called the blue (B), green (G), and red (R) responsesbecause the three film component sensitivities fall in the regions ofthe spectrum that are so described. FIG. 1 illustrates the three filmresponse sensitivities. We observe that the blue component is mostsensitive, the red component is least sensitive, and the green componentis intermediate.

FIG. 2 shows the relative spectral power distribution of a 3200 Kincandescent (i.e., tungsten) light source. When this spectral power isindependently weighted by each of the three spectral sensitivities, theresulting blue (B), green (G), and red (R) responses are proportional tothe areas under the respective product curves. The necessary conditionfor the required achromatic reproduction of a non-selectively reflectingsurface is that B:G:R is in the proportions of 1:1:1. This establishesthe “white point” for the film exposure. We note that the spectral powerof the 3200 K incandescent source increases monotonically from lower tohigher wavelengths, a variation that is in opposition to that of theareas under the three sensitivity curves versus wavelength. Clearly thebalance of film sensitivities has been set to give three equal responsesunder a 3200 K incandescent light source.

As an essential component of the computation for the desired lightsource spectral distribution, note that the blue sensitivity of the filmextends below 400 nm into the ultraviolet region of the spectrum. If thedesign were to proceed with the inherent blue film sensitivity,nonvisible radiation would influence the white point balance. But wealso note that photographic lenses attenuate short wavelength radiationto a greater or lesser extent in the ultraviolet and sometimes well intothe violet and blue regions of the visible spectrum. The effect dependson the various glasses used in manufacturing of a particularphotographic camera lens and on the various lens coatings commonly usedto reduce surface reflections at the various glass surfaces within thelens. The best we can do when designing a photographic light source tobe used with a particular film is to make an average correction that isreasonably representative for most lenses. In this instance, we selectthe average lens spectral transmittance that is used in the standardANSI PH2.3-1972. This lens transmittance (see FIG. 3) greatly reducesthe blue response of the film to UV in a light source. The product ofthe blue film sensitivity and the typical average lens transmittanceresults in an effective blue film response as shown by the dashed linein FIG. 1. It is this modified blue response that is used in the LEDdesign process. If the lens transmittance did not attenuate in thevisible spectrum, this would be a moot point for an LED system in whichthe LED system does not radiate in the ultraviolet. However, theattenuation in the blue end of the visible spectrum above 400 nm willaffect the white point balance.

The final acceptance of an LED or most other types of light sources forphotography and cinematography cannot be set by computational ormeasurement means. It anticipated and expected that each light will haveminor and subtle effects on the film exposure. These only can bedetermined by film tests using the actual or typical scenes. Thelighting director will look at the resulting photography and make adecision of acceptability. If it is not quite to his liking, a weakcolor compensating and/or light balancing filter will be placed in frontof the light. At the same time another lighting director may prefer thelight without the filter(s) rather than the light changed by the filter.Because this is an issue of human preference, we do not expect universalagreement on the exact characteristics of a preferred light. Otherissues such as the variation of any particular lens from the averagetypical lens transmittance will have a small effect on the performanceof the light interacting with the film response. Many other minorfactors are known and controlled to a greater or lesser extent. Examplesof such factors would be deviations in the particular batch of filmstock, the developing process, nonlinearities in the photographicprocess, etc. Consequently, some small variation in the B:G:R ratiocharacterizing an acceptable light source is expected and will not besignificant providing that it is not excessive. When making finalevaluative judgments of the color balance, there may be smalldisplacements in the target design ratio due to such factors as the bluecutoff of the actual lens used in the evaluation.

What we have invented is a phosphor blend and an LED light sourcedesigned for use in conjunction with color film balanced for 3200 Kincandescent (tungsten) lighting. The ratio of the three colorcomponents of the film, B:G:R, when directly exposed through a nominalphotographic lens, is 1:1:1. The acceptable variability for thisinvention is defined at ±20% for each of the three numerical values.Preferably, the LED light source has a B:G:R ratio for a 3200K tungstenbalanced color film of X:Y:Z when directly exposed through a nominalphotographic lens, wherein X, Y and Z each have a value from 0.90 to1.10.

FIG. 4 shows an exemplary LED light source according to an embodiment ofthis invention for use with 3200 K tungsten balanced color film. TheB:G:R ratio is 1.00:1.09:0.99. While visible metrics are irrelevant forthis invention in its intended application, such values may aid isvisualizing this light source. The correlated color temperature is 3257K while the general color rendering index is 95. In a preferredembodiment, the LED sources use blue-emitting LED chips with phosphorembedded domes that provide the full required spectral range.Blue-emitting LEDs with dominate wavelengths in the range of 455-465 nmare typically used and the phosphor embedded domes contain afour-component blend of phosphors.

The four phosphor components in the blend include (1) aeuropium-activated calcium magnesium chlorosilicate phosphor, preferablyhaving a composition represented by the formula, Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺,and having a peak emission at about 500-510 nm; (2) a europium-activatedstrontium calcium silicon nitride phosphor, preferably having acomposition represented by the formula, Sr_(2-x)Ca_(x)Si₅N₈:Eu²⁺, andhaving a peak emission at about 655-665 nm; (3) a europium-activatedcalcium silicon nitride phosphor, preferably having a compositionrepresented by the formula, Ca₂Si₅N₈:Eu²⁺, and having a peak emission atabout 610-620 nm; and (4) a cerium-activated yttrium aluminum garnetphosphor, preferably having a composition represented by the formula,Y₃Al₅O₁₂:Ce³⁺, and having a peak emission at about 560-570 nm. Thepreferred phosphor blend in Table I was developed to couple with thespectral response of common tungsten balanced films. Depending on thegeometry of the part containing the phosphor blend (dome volumeconverted package) the concentration of the phosphor blend in thesilicone matrix will vary. Typical values are 5-10 weight percent (wt %)phosphor blend in the silicone matrix.

TABLE I Preferred composition of the 3200K tungsten phosphor blend.Phosphor dominant Phosphor Wt % wavelength (nm) Ca₈Mg(SiO₄)₄Cl₂: Eu²⁺68.2 524 Ca₂Si₅N₈: Eu²⁺ 17.5 594 Sr_(2−x)Ca_(x)Si₅N₈: Eu²⁺ 4.5 608Y₃Al₅O₁₂: Ce³⁺ 9.8 573

FIG. 5 shows the chromaticity point of the LED light source on the 1931CIE Chromaticity Diagram where correlated color temperature lines arelabeled in kelvin. The chromaticity, the correlated color temperature,and the general color rendering index are not given tolerances becausethey are dependent variables; B, G, and R are the independent variables.FIG. 6 shows the individual emission curves for the four phosphorscomprising the blend. The phosphors are excited by the blue lightemission from the blue LED and the light emitted by the phosphorscombines with the remaining blue emission from the LED to produce thedesired spectral output.

FIG. 7 is a top view of an array of LED light sources according to thisinvention. The LED light sources 10 are mounted on a circuit board 20 ina 5×5 array. The circuit board 20 is fitted with an electrical connector15 for supplying power to the LED light sources. A cross-sectionalillustration of one of the LED light sources 10 is shown in FIG. 8. TheLED die 25 is mounted to the circuit board 20 and is covered by aphosphor-filled dome 30. The dome contains the 4-component phosphorblend. The space 35 between the LED die 25 and the dome 30 is preferablyfilled with a transparent silicone resin.

While there have been shown and described what are at present consideredto be preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims.

We claim:
 1. A phosphor blend for an LED light source comprising fromabout 7 to about 12 weight percent of a cerium-activated yttriumaluminum garnet phosphor, from about 3 to about 6 weight percent of aeuropium-activated strontium calcium silicon nitride phosphor, fromabout 15 to about 20 weight percent of a europium-activated calciumsilicon nitride phosphor, and from about 55 to about 80 weight percentof a europium-activated calcium magnesium chlorosilicate phosphor. 2.The phosphor blend of claim 1 wherein the blend comprises from about 8to about 10.5 weight percent of the cerium-activated yttrium aluminumgarnet phosphor, from about 4 to about 5 weight percent of theeuropium-activated strontium calcium silicon nitride phosphor, fromabout 16.5 to about 18.5 weight percent of the europium-activatedcalcium silicon nitride phosphor, and from about 65 to about 75 weightpercent of the europium-activated calcium magnesium chlorosilicatephosphor.
 3. The phosphor blend of claim 1 wherein the blend comprisesabout 10 weight percent of the cerium-activated yttrium aluminum garnetphosphor, about 4.5 weight percent of the europium-activated strontiumcalcium silicon nitride phosphor, about 17.5 weight percent of theeuropium-activated calcium silicon nitride phosphor, and about 68 weightpercent of the europium-activated calcium magnesium chlorosilicatephosphor.
 4. The phosphor blend of claim 1 wherein the cerium-activatedyttrium aluminum garnet phosphor has a composition represented by aformula Y₃Al₅O₁₂:Ce³⁺, the europium-activated strontium calcium siliconnitride phosphor has a composition represented by a formulaSr_(2-x)Ca_(x)Si₅N₈:Eu²⁺, the europium-activated calcium silicon nitridephosphor has a composition represented by a formula Ca₂Si₅N₈:Eu²⁺, andthe europium-activated calcium magnesium chlorosilicate phosphor has acomposition represented by a formula Ca₈Mg(SiO₄)₄Cl₂:Eu²⁺.